The present invention relates to electronic semiconductor devices and methods of fabrication, and, more specifically, to gallium arsenide bipolar devices that are formed with gallium arsenide FETs in a single substrate.
Semiconductor devices made of gallium arsenide are preferred over devices made of silicon for high frequency applications due to the higher mobility of carriers in gallium arsenide. In addition, most gallium arsenide technologies are expected to exhibit improved radiation hardness for total dose ionizing radiation over silicon Nevertheless, gallium arsenide material and fabrication technology lag far behind that of silicon. Indeed, gallium arsenide MESFET integrated circuits with 6,000 gates have been fabricated (for example, Terada et al, A 64 K GaAs Gate Array, 1987 ISSCC Dig. Tech Papers 144), but precise control of device parameters such as threshold voltage for larger scale integration continues to be a concern. Moreover, While GaAs MESFET technology has achieved subnanosecond switching speeds, problems associated with noise margins, operating temperature and device packing density continue to present limitations. Similarly, HEMTs, which use the two dimensional electron gas at a heterojunction of gallium arsenide and aluminum gallium arsenide, provide fast devices but suffer from lack of precise control of device parameters; see Mimura et al, High Electron Mobility Transistors for LSI Circuits, 1983 IEDM Tech. Digest 99. In contrast, JFET technology offers more fabrication flexibility without significant loss in device performance.
Bipolar transistors offer several potential advantages over the field-effect transistor for high speed applications. Higher operating speeds should be possible because the vertical dimensions (layers) of a bipolar structure may be smaller than the lithographically limited horizontal dimensions of the FET thereby reducing the dimensions of critical current paths. In addition, tho threshold voltage for the commonly employed bipolar devices is largely dependent on the base bandgap whereas the FET threshold voltage depends on device geometry and doping levels which are much harder to control. Furthermore, the bipolar process offers improved temperature performance making it an ideal candidate for high temperature/high radiation environments. For background, see M. Nowogrodzki, Advanced III-IV semiconductor Materials Technology Assessment 1984, which is hereby incorporated by reference.
The recognized advantages of GaAs bipolar technology have been recently realized in the fabrication of a 4 K gate array which employed heterojunction inverted transistor integrated logic (HI2L) (Yuan et al, A 4 K GaAs Gate Array, 1986 ISSCC Dig. Tech. Papers 74). However, since HI2L transistors all have their emitters grounded, a larger scale circuit fabricated with only HI2L devices has a relatively high power consumption compared to a FET equivalent Accordingly, it would be desirable to exploit the advantages of both HI2L and FET GaAs technology as the need for higher levels of integration increases.